The present invention relates to a cutting tool with increased tool life when machining grey cast iron.
In the description of the background of the present invention that follows reference is made to certain structures and methods, however, such references should not necessarily be construed as an admission that these structures and methods qualify as prior art under the applicable statutory provisions. Applicants reserve the right to demonstrate that any of the referenced subject matter does not constitute prior art with regard to the present invention.
Cutting tools having cutting edges formed of a superhard abrasive such as a cubic boron nitride (CBN) based material are manufactured by powder metallurgical techniques and are to an increasing extent used for the machining of cast iron and hardened steel. Several types of CBN cutting tools are known, the majority consisting of a CBN tip that has been brazed onto a cemented carbide insert. Others have the CBN sintered directly to a cemented carbide backing of sufficient thickness to produce an insert while yet others consist of mainly CBN without any cemented carbide backing.
Subjecting an insert with a brazed on CBN/cemented carbide tip to temperatures over 700xc2x0 C. will result in destroying the braze, but for inserts without any braze joint there is no such limitation.
When machining hardened steels, e.g., ball bearing steels, chemical dissolution of the tool material is an important part of the wear mechanism, provided that the tool material has sufficient hot hardness to resist plastic deformation. For that reason the CBN materials used for such work materials usually contain a high fraction of secondary phases with low chemical solubility in steel. In WO 98/28464 it is shown in the example that applying a coating including a MTCVD-TiCN and a CVD-Al2O3-layer to such a low CBN tool material, substantial advantages in tool life can be achieved when machining hardened ball bearing steel. The CVD-coating is applied to a substrate comprising cemented carbide and at least one CBN body whereby the cemented carbide constitutes the main part of the substrate.
When machining grey cast iron, the dominating wear mechanism is of abrasive nature, and consequently due to the high hardness of CBN, the most suitable material for grey cast iron is that of high CBN content, generally more than 60%, often over 80%. An example of such a material is disclosed in U.S. Pat. No. 5,326,380. Inserts of this type are usually produced as a layer of CBN-containing material bonded through sintering to a cemented carbide backing, or a solid CBN-compact thus having no braze joint on the finished insert that can be damaged at normal CVD coating temperatures.
The tool comprises a CBN-containing body with more than 60 vol % CBN and a coating. The coating consists of at least one layer of metal nitride, carbonitride or carbide with the metal elements selected from at least one of Ti, Nb, Hf, V, Ta, Mo, Zr, Cr, W and Al and at least one layer of Al2O3. The coating is a CVD coating deposited at 900xc2x0 C. or higher.